1. Field of the Invention
The present invention relates to a radiographic image capturing system, and in particular, to a radiographic image capturing system that can carry out fluoroscopic imaging in which the capturing of radiographic images is carried out continuously.
2. Related Art
Radiation detectors such as FPDs (Flat Panel Detectors), in which a radiation-sensitive layer is disposed on a TFT (Thin Film Transistor) active matrix substrate and that can convert radiation directly into digital data have been put into practice in recent years. Portable radiographic image capturing devices (hereinafter also called “electronic cassettes”), that capture radiographic images expressed by irradiated radiation by using the radiation detector, are being put into practice. As compared with a radiographic image capturing device that uses a conventional X-ray film or imaging plate, a radiographic image capturing device that uses the radiation detector has the advantages that images can be confirmed immediately, and fluoroscopic imaging (video image capturing), in which the capturing of radiographic images is carried out continuously, also can be carried out. As methods of converting radiation at the radiation detector, there are an indirect conversion method that, after converting radiation into light at a scintillator, converts the light into charges at a semiconductor layer of photodiodes or the like, and a direct conversion method that converts radiation into charges at a semiconductor layer of amorphous selenium or the like, and the like. There exist various types of materials that can be used at the semiconductor layer in these respective methods.
As image capturing methods for fluoroscopic imaging, there are a method of capturing images at a predetermined frame rate while irradiating radiation continuously from a radiation source (continuous irradiation), and a method of, while irradiating radiation in the form of pulses synchronously with the frame rate (pulse irradiation), capturing images synchronously with the irradiation of the radiation. With pulse irradiation, the radiation can be irradiated for the time needed for imaging, and the amount of radiation that the patient is exposed to can be suppressed as compared with continuous irradiation, and there is therefore the advantage that the irradiated amount per unit time can be increased. However, with pulse irradiation, there is the need to synchronize the timing of irradiating the radiation from the radiation source and the timing of the image capturing at the radiation detector.
Japanese Patent Application Laid-Open (JP-A) No. 2009-136481 discloses a technique in which a switch for switching between continuous irradiation and pulse irradiation is provided. In imaging using C arm, when the C arm is rotated and positioning of the imaged region is carried out, the form of irradiation is switched to pulse irradiation by the switch. When capturing diagnostic images, the form of irradiation is switched to continuous irradiation by the switch, and image capturing is carried out.
JP-A No. 2009-186439 discloses, in a wireless X-ray fluoroscopic system that is physically divided into an exposure unit and a sensor unit, a technique of generating beacon signals at a period that is associated with the frame rate of image capturing, and wirelessly synchronizing the irradiation timing and the image capturing timing.
In pulse irradiation, because the irradiation time is short, the respective images become frame-advanced images having stopped motion. This trend occurs in particular in cases in which the frame rate is low, because the image capturing interval is large and after-images of the eyes also disappear.
Thus, fluoroscopic images having smooth motion may not be captured by pulse irradiation in a case in which the frame rate is low.
Note that, in the techniques of JP-A Nos. 2009-136481 and 2009-186439 as well, the capturing of fluoroscopic images having smooth motion is difficult in cases in which the frame rate is low.